共查询到20条相似文献,搜索用时 15 毫秒
1.
An Australian hard wheat flour–water dough has been characterised using parallel plate and capillary rheometers over an extensive
range of apparent shear rates (10 − 3–103 s − 1) relevant to process conditions. Torsional measurements showed that the shear viscosity of the dough increased with strain
to a maximum value and then decreased, suggesting a breakdown of the dough structure. Both torsional and capillary experiments
revealed the shear-thinning behaviour of the dough. The wall slip phenomenon in capillary rheometry was investigated and found
to be diameter dependent and occurred at a critical shear stress of approximately 5–10 kPa. A two-regime power law behaviour
was observed, with the power law index approximately 0.3 in the low shear rate range increasing to 0.67 in the high shear
rate range. Pressure fluctuation was observed in the capillary data and increased with shear rate, in particular, at shear
rates approaching 104 s − 1. The results demonstrate that capillary rheometry is a viable means of rheologically testing dough at high shear rates provided
pressure fluctuation is carefully monitored and capillary rheometry corrections, including wall slip, are accounted for. 相似文献
2.
Christopher J. Dimitriou Laura Casanellas Thomas J. Ober Gareth H. McKinley 《Rheologica Acta》2012,51(5):395-411
We explore the behavior of a wormlike micellar solution under both steady and large amplitude oscillatory shear (LAOS) in
a cone–plate geometry through simultaneous bulk rheometry and localized velocimetric measurements. First, particle image velocimetry
is used to show that the shear-banded profiles observed in steady shear are in qualitative agreement with previous results
for flow in the cone–plate geometry. Then under LAOS, we observe the onset of shear-banded flow in the fluid as it is progressively
deformed into the non-linear regime—this onset closely coincides with the appearance of higher harmonics in the periodic stress
signal measured by the rheometer. These harmonics are quantified using the higher-order elastic and viscous Chebyshev coefficients
e
n
and v
n
, which are shown to grow as the banding behavior becomes more pronounced. The high resolution of the velocimetric imaging
system enables spatiotemporal variations in the structure of the banded flow to be observed in great detail. Specifically,
we observe that at large strain amplitudes (γ
0 ≥ 1), the fluid exhibits a three-banded velocity profile with a high shear rate band located in-between two lower shear rate
bands adjacent to each wall. This band persists over the full cycle of the oscillation, resulting in no phase lag being observed
between the appearance of the band and the driving strain amplitude. In addition to the kinematic measurements of shear banding,
the methods used to prevent wall slip and edge irregularities are discussed in detail, and these methods are shown to have
a measurable effect on the stability boundaries of the shear-banded flow. 相似文献
3.
The rheological characterisation of concentrated shear thickening materials suspensions is challenging, as complicated and occasionally discontinuous rheograms are produced. Wall slip is often apparent and when combined with a shear thickening fluid the usual means of calculating rim shear stress in torsional flow is inaccurate due to a more complex flow field. As the flow is no longer “controlled”, a rheological model must be assumed and the wall boundary conditions are redefined to allow for slip. A technique is described where, by examining the angular velocity response in very low torque experiments, it is possible to indirectly measure the wall slip velocity. The suspension is then tested at higher applied torques and different rheometer gaps. The results are integrated numerically to produce shear stress and shear rate values. This enables the measurement of true suspension bulk flow properties and wall slip velocity, with simple rheological models describing the observed complex rheograms. 相似文献
4.
Shear and extensional viscosities and wall slip are determined simultaneously under extrusion processing conditions using
an on-line rheometer. Because it is not possible to independently control flow rate and temperature, classical methods for
interpretation of capillary data cannot be used with on-line rheometry. This limitation is overcome using computational optimization
to fit parameters in a flow model. This consists of three parts, representing shear viscosity, extensional viscosity, and
wall slip. Three-parameter, power law forms, based on local instantaneous deformation rates and including temperature dependence,
are used for each, and analytic solutions applied for entry flow and flow in the capillary. For entry flow, the Cogswell–Binding
approach is used, and for developed flow in the capillary a solution incorporating wall slip is derived. The rheometer, with
interchangeable capillaries, is mounted in place of the die on a rubber profile extrusion line. Pressure drops and temperatures
for extrusion of an EPDM rubber through 2 mm diameter capillaries of length 0, 2, 3, 4, and 5 mm are logged and flow rates
determined for a range of extruder speeds (5 to 20 rpm). Pressures ranged from 60 to 75 bar and temperatures from 86 to 116 °C.
Mean flow velocity in the capillaries was between 5 × 10−3 and 5 × 10−1 m s−1. The nine material parameters are optimized for best fit of the analytic pressure drops to experimental data, using about
100 data points, with the Levenberg–Marquardt method. It is concluded that flow is dominated by extension and wall slip. Shear
flow appears to play little part. The slip model indicates that slip velocity increases much more rapidly than the wall shear
stress (in the range 0.5–1 MPa) and decreases with temperature for a given stress level. Results for the (uniaxial) extensional
viscosity represent an engineering approximation to this complex phenomenon at the high strains (approximately 200) and high
extension rates (up to 800 s−1) applying in the extrusion. Results indicate a slight extension hardening and a decrease with temperature. Results are put
into the context of the available studies in the literature, which, particularly with regard to wall-slip and extensional
flow, consider conditions far removed from those applying in industrial extrusion. The present methods provide a powerful
means for flow characterization under processing conditions, providing data suitable for use in computer simulations of extrusion
and optimization of die design. 相似文献
5.
Arturo F. Méndez-Sánchez M. Rosario López-González V. Hugo Rolón-Garrido José Pérez-González Lourdes de Vargas 《Rheologica Acta》2003,42(1-2):56-63
The rheological behavior of a cetylpyridinium chloride 100 mmol l–1/sodium salicylate 60 mmol l–1 aqueous solution was studied in this work under homogeneous (cone and plate) and non-homogeneous flow conditions (vane-bob
and capillary rheometers), respectively. Instabilities consistent with non-monotonic flow curves were observed in all cases
and the solution exhibited similar behavior under the different flow conditions. Hysteresis and the sigmoidal flow curve suggested
as characteristic of systems that show constitutive instabilities were observed when running cycles of increasing and decreasing
stress or shear rate, respectively. This information, together with a detailed determination of steady states at shear stresses
close to the onset of the instabilities, allowed one to show unequivocally that "top and bottom jumping" are the mechanisms
to trigger the instabilities in this micellar system. It is shown in addition that there is not a true plateau region in between
the "top and bottom jumping". Finally, the flow behavior beyond the upturn seemed to be unstable and was found accompanied
by an apparent violation of the no-slip boundary condition. 相似文献
6.
An empirical correlation for the onset of turbulence in physiological pulsatile flow is presented. We pumped three different
test fluids of kinematic viscosity 0.008–0.035 cm2/s through four straight tubes 0.4–3.0 cm in diameter. A Scotch yoke mechanism provided an oscillatory sine wave flow component
of known stroke volume and frequency. We adjusted the mean flow independently until we detected signal instabilities from
hot film wall shear stress probes.
The critical peak Reynolds number was found to correlate with the Womersley parameter and the Strouhal number as a power law
function with a root-mean-square (rms) error of 15.2%. Experimental measurements of the laminar velocity profile are compared
to theoretical predictions from Poiseuille’s law and Womersley’s solution.
Received: 30 October 1995/Accepted: 7 April 1997 相似文献
7.
8.
Norhayani Othman Bashar Jazrawi Parisa Mehrkhodavandi Savvas G. Hatzikiriakos 《Rheologica Acta》2012,51(4):357-369
The wall slip and melt fracture behaviour of several commercial polylactides (PLAs) as well as their rheological properties
under shear and extensional have been investigated. The PLAs have had weight-average molecular weights in the range of 104–105 g/mol and studied in the temperature range of 160–200°C. The solution properties and linear viscoelastic behaviour of melts
indicate linear microstructure behaviour. PLAs with molecular weights greater than a certain value were found to slip, with
the slip velocity to increase with decrease of molecular weight. The capillary data were found to agree well with linear viscoelastic
envelope once correction for slip effects was applied. The onset of melt fracture for the high molecular weight PLAs was found
to occur at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature.
Addition of 0.5 wt.% of a polycaprolactone (PCL) into the PLA that exhibits melt fracture was found to be effective in eliminating
and delaying the onset of melt fracture to higher shear rates. This is due to significant interfacial slip that occurs in
the presence of PCL. 相似文献
9.
The near-wall transport characteristics, inclusive of mass transfer coefficient and wall shear stress, which have a great
effect on gas–liquid two-phase flow induced internal corrosion of low alloy pipelines in vertical upward oil and gas mixing
transport, have been both mechanistically and experimentally investigated in this paper. Based on the analyses on the hydrodynamic
characteristics of an upward slug unit, the mass transfer in the near wall can be divided into four zones, Taylor bubble nose
zone, falling liquid film zone, Taylor bubble wake zone and the remaining liquid slug zone; the wall shear stress can be divided
into two zones, the positive wall shear stress zone associated with the falling liquid film and the negative wall shear stress
zone associated with the liquid slug. Based on the conventional mass transfer and wall shear stress characteristics formulas
of single phase liquid full-pipe turbulent flow, corrected normalized mass transfer coefficient formula and wall shear stress
formula are proposed. The
calculated results are in good agreement with the experimental data. The shear stress and the mass transfer coefficient in
the near wall zone are increased with the increase of superficial gas velocity and decreased with the increase of superficial
liquid velocity. The mass transfer coefficients in the falling liquid film zone and the wake zone of leading Taylor bubble
are lager than those in the Taylor bubble nose zone and the remaining liquid slug zone, and the wall shear stress associated
falling liquid film is larger than that associated the liquid slug. The mass transfer coefficient is within 10−3 m/s, and the wall shear stress below 103 Pa. It can be concluded that the alternate wall shear stress due to upward gas–liquid slug flow is considered to be the major
cause of the corrosion production film fatigue cracking. 相似文献
10.
Souzanna Sofou Edward B. Muliawan Savvas G. Hatzikiriakos Evan Mitsoulis 《Rheologica Acta》2008,47(4):369-381
Bread dough (a flour–water system) has been rheologically characterized using a parallel-plate, an extensional, and a capillary
rheometer at room temperature. Based on the linear and nonlinear viscoelastic and viscoplastic data, two constitutive equations
have been applied, namely a viscoplastic Herschel–Bulkley model and a viscoelastoplastic K–BKZ model with a yield stress.
For cases where time effects are unimportant, the viscoplastic Herschel–Bulkley model can be used. For cases where transient
effects are important, it is more appropriate to use the K-BKZ model with the addition of a yield stress. Finally, the wall
slip behavior of dough was studied in capillary flow, and an appropriate slip law was formulated. These models characterize
the rheological behavior of bread dough and constitute the basic ingredients for flow simulation of dough processing, such
as extrusion, calendering, and rolling. 相似文献
11.
G. H. Meeten 《Rheologica Acta》2008,47(8):883-894
The rheology and slip of a dry shaving foam are investigated using squeeze-flow and rotating-vane methods. Constant-force
squeeze flow between planar surfaces is used to study the effect of surface roughness on slip and to obtain the yield stress.
Non-slip vane measurements are used to obtain the linear shear viscosity and elasticity at small strains, and the yield stress
and strain at large strains. Data are compared with the small-strain Maxwell and Kelvin–Voigt linear-viscoelastic models.
An apparent dependence of the yield stress and elasticity on the rotational speed of the vane is shown to result from time-dependent
rheological parameters as the foam ages. The effect of viscosity in the pre-yield region may give an erroneous identification
of yield. 相似文献
12.
When the flow behaviour of fluids is investigated with capillary-or rotational rheometers, adhesion of the fluid to the wall is normally one of the boundary conditions. For many fluids, especially for suspensions, this assumption is not valid. These fluids tend to slip at the wall. Therefore the normal evaluation of rheometer measurements leads to apparent but not compatible flow functions. The flow behaviour of these fluids can be characterized with two material functions which describe separately slipping in the boundary layer and shearing within the fluid. Only if both functions are known, correct predictions of flow processes are possible. A simple equipment to separate the shear function and the slip function is described.List of symbols Y*
apparent shear rate
- Y
w
*
apparent wall shear rate
- Yw
wall shear rate corrected with Rabinowitsch and Weissenberg correction
- Ys
reduced shear rate (slip corrected)
- Yws
reduced wall shear rate (slip corrected)
- * (r)
velocity distribution in a capillary
- G
slip velocity (at the wall)
- * (r)
velocity distribution in a capillary (without slip)
-
shear stress
-
w
wall shear stress
- VS
total volume rate
- VG
shear volume rate
- VG
slip volume rate
-
p
1
pressure in the reservoir channel of the capillary rheometer
-
p
0
athmospheric pressure
-
L
capillary length
-
R
capillary radius 相似文献
13.
A two-stage Tikhonov regularisation procedure has been used to obtain rheological properties for a high internal phase emulsion
from gap-dependent steady-state parallel plate shear data. This method is beneficial in that it can convert the steady shear
data into rheological property functions. The built-in regularisation parameters of the method are able to keep noise amplification
under control. The two-stage method is able to obtain not only the shear stress–shear rate function but also the apparent
slip velocity as a function of wall shear stress. The method is such that it obtains the rheological functions over the maximum
range of shear rate covered by the data. The results obtained using the new method are compared to those obtained using the
vane geometry with good agreement being observed. 相似文献
14.
The thickening properties of many commercial thickeners are difficult to measure because of wall slip artefacts. Here we
report a series of experiments on a typical thickener where these artefacts have been successfully eliminated. As a result,
complete, steady-state flow-curves of aqueous Carbopol 980 (the toxicologically preferred version of the older and more well-known
Carbopol 940) dispersions are reported for a range of concentrations of 0.045–1.0 wt%. The vane-and-basket flow geometry was
used to avoid slip problems at low shear stress, with the geometry housed in a TA AR1000-N controlled-stress rheometer, whilst
a Haake RV2 viscometer with an SV2P and MV2P concentric-cylinder geometries were used at higher shear rates. The flow-curves
obtained show a smooth but steep transition from a very high Newtonian viscosity at low shear stress to a much lower viscosity
at high shear stress. No real yield stresses were detected, but the higher shear rate results can be fitted to the Herschel-Bulkley
model, which assumes an apparent yield stress. The various model parameters are displayed as a function of Carbopol concentration.
Received: 29 November 2000/Accepted: 26 February 2001 相似文献
15.
Francisco Rodríguez-González José Pérez-González Lourdes de Vargas Benjamín M. Marín-Santibáñez 《Rheologica Acta》2010,49(2):145-154
The continuous extrusion of a metallocene linear low-density polyethylene through a transparent capillary die with and without
slip was analyzed in this work by rheometrical measurements and particle image velocimetry (PIV). For this reason, a comparison
was made between the rheological behaviors of the pure polymer and blended with a small amount of fluoropolymer polymer processing
additive. Very good agreement was found between rheometrical and PIV measurements. The pure polymer exhibited stick-slip instabilities
with nonhomogeneous slip at the die wall, whereas the blend showed stable flow. The slip velocity was measured directly from
the velocity profiles and was negligible for the pure polymer before the stick-slip but increased monotonously as a function
of the shear stress for the blend. The flow curves and the slip velocity as a function of the shear stress deviated from a
power law and were well fitted by continuous “kink” functions. Comparison of PIV data with rheometrical ones permitted a direct
proof of the basic assumption of the Mooney theory. Finally, the analysis of the velocity profiles showed that there is a
maximum in the contribution of slip to the average fluid velocity, which is interpreted as the impossibility for the velocity
profile to become plug like in the presence of shear thinning. 相似文献
16.
Babak Derakhshandeh Savvas G. Hatzikiriakos Chad P. J. Bennington 《Rheologica Acta》2010,49(11-12):1127-1140
Conventional rheometry coupled with local velocity measurements (ultrasonic Doppler velocimetry) are used to study the flow behaviour of various commercial pulp fibre suspensions at fibre mass concentrations ranging from 1 to 5 wt.%. Experimental data obtained using a stress-controlled rheometer by implementing a vane in large cup geometry exhibits apparent yield stress values which are lower than those predicted before mainly due to existence of apparent slip. Pulp suspensions exhibit shear-thinning behaviour up to a high shear rate value after which Newtonian behaviour prevails. Local velocity measurements prove the existence of significant wall slippage at the vane surface. The velocimetry technique is also used to study the influence of pH and lignin content on the flow behaviour of pulp suspensions. The Herschel–Bulkley constitutive equation is used to fit the local steady-state velocity profiles and to predict the steady-state flow curves obtained by conventional rheometry. Consistency between the various sets of data is found for all suspensions studied, including apparent yield stress, apparent wall slip and complete flow curves. 相似文献
17.
The flow-induced microstructure of a mesophase pitch was studied within custom-made dies for changing wall shear rates from
20 to 1,100 s − 1, a flow scenario that is typically encountered during fiber spinning. The apparent viscosity values, measured at the nominal
wall shear rates ranging from 500 to 2,500 s − 1 using these dies, remain fairly constant. The microstructure was studied in two orthogonal sections: r–θ (cross section) and r–z (longitudinal mid plane). In these dies, the size of the microstructure gradually decreases toward the wall (to as low as
a few micrometers), where shear rate is highest. Furthermore, as observed in the r–θ plane of the capillary, for a significant fraction of the cross section, discotic mesophase has a radial orientation. Thus,
the directors of disc-like molecules were aligned in the vorticity (θ) direction. As confirmed from the microstructure in the r–z plane, most of the discotic molecules remain nominally in the flow plane. Orientation of the pitch molecules in the shear
flow conditions is consistent with that observed in controlled low-shear rheometric experiments reported earlier. Microstructral
investigation suggests that the radial orientation of carbon fibers obtained from a mesophase pitch originates during flow
of pitch through the die. 相似文献
18.
The use of a sliding plate rheometer (SPR) to determine the first normal stress difference of molten polymers and elastomers
at high shear rates is demonstrated. The simple shear flow in this instrument is not subject to the flow instabilities that
limit the use of rotational rheometers to shear rates often below 1 s−1. However, issues of secondary flow and wall slip must be addressed to obtain reliable data using an SPR. A highly entangled,
monodisperse polybutadiene and a commercial polystyrene were the polymers studied. The inclusion of the polystyrene made it
possible to compare data with those obtained by Lodge using a stressmeter, which is an instrument based on the measurement
of the hole pressure. The data from the two instruments are in good agreement and are also close to the predictions of an
empirical equation of Laun based on the storage and loss moduli. 相似文献
19.
Enric Santanach Carreras Nadia El Kissi Jean-Michel Piau Fabrice Toussaint Sophie Nigen 《Rheologica Acta》2006,45(3):209-222
In the present work, the effects of pressure on the viscosity and flow stability of four commercial grade polyethylenes (PEs)
have been studied: linear-low-density polyethylene copolymer, high-density polyethylene, metallocene polyethylenes with short-chain
branches (mPE-SCB), and metallocene polyethylenes with long chain branching (mPE-LCB). The range of shear rates considered
covers both stable and unstable flow regimes. “Enhanced exit-pressure” experiments have been performed attaining pressures
of the order of 500×105 Pa at the die exit. The necessary experimental conditions have been clearly defined so that dissipative heating can be neglected
and pressure effects isolated. The results obtained show an exponential increase in both shear and entrance-flow pressure
drop with mean pressure when shear rate is fixed and as long as flow is stable. These pressure effects are described by two
pressure coefficients, βS under shear and, βE under elongation, that are calculated using time–pressure superposition and that are independent of mean pressure and flow
rate. For three out of four PE, pressure coefficient values can be considered equal under shear and under elongation. However,
for the mPE-LCB, the pressure coefficient under elongation is found to be about 30% lower than under shear. Flow instabilities
in the form of oscillating flows or of upstream instabilities appear at lower shear rates as mean pressure increases. Nevertheless,
the critical shear stress at which they are triggered remains independent of mean pressure. Moreover, it is found that the
βS values obtained for stable flows do not differ much from the values obtained during upstream instability regimes, and differ
really from pressure effects observed under oscillating flow and slip conditions. 相似文献
20.
Planar contraction flows of non-Newtonian fluids with integral constitutive models are studied to investigate the problem
of numerical breakdown at high Weissenberg or Debrorah numbers. Spurious shear stress extrema are found on the wall downstream
of the re-entrant corner for both sharp and rounded corners. Moreover, a non-monotonic relation between shear stress and strain
rate is found when the Deborah number limit is approached, which correlates with these shear extrema. This strongly suggests
that non-monotonicity between shear stress and strain rate may be responsible for the Deborah number limit problem in contraction
flow simulations. This non-monotonicity is caused by the inaccuracy of the quadrature, using constitutive equations that do
not have shear stress maxima when exactly evaluated. This conclusion agrees with recent analytical findings by others that
inaccuracy of the integration along the streamlines – either by numerical integration or asymptotic approximation – makes
the problem ill-conditioned, with spurious growth occurring on the wall downstream of the re-entrant corner.
Received: 5 March 1999/Accepted: 1 September 1999 相似文献